1. Field of the Invention
The invention is directed to fat and wax-based ink compositions for ink-jet printing on edible substrates, to methods of ink-jet printing on edibles with the inks, and to the edibles made. The inks and methods described herein may be employed for forming printed edible products as diverse as pharmaceutical pills and tablets to pet foods. However, the invention has particular utility for printing on confectionery, including, without limitation, chocolate bars and tableted products, jelly beans, toffees and chewing gums, and particularly for printing high resolution and high definition images on edible substrate surfaces that are difficult to print on using conventional technology. Such surfaces include, without limitation, the non-planar, non-porous, hydrophobic surfaces of M&M's® Milk Chocolate and Peanut Chocolate Candies, which have a sugar shell with a carnauba wax polish coating.
2. Description of the Related Art
It is known to print identifying or decorative images on edibles. The methods generally relied upon to print on small pieces of confectionery or on pharmaceuticals include rotogravure, flexography, silk screen and pad printing. Each of these methods has its own drawbacks, and each of them requires contacting the piece to be printed.
The present technology for printing on M&M's® Milk Chocolate and Peanut Chocolate Candies is by a contact printing process utilizing an offset roller, in what is referred to herein as rotogravure printing. The rotogravure system is limited in the number of colors that can be applied to a substrate. Traditionally, one color is printed, and with modifications two or three colors may be applied, but full color printing on edibles is not possible. The rotogravure roller is also limited in the surface area of a non planar piece that it can print. As with other contact methods of printing, there is a danger of the rotogravure roller crushing the edible articles being printed. Print designs cannot be changed or modified easily using the rotogravure method, because each new design must be engraved on a roller.
Silk screen and pad printing, although capable of forming reasonably high resolution and high definition images, cannot deliver the dots-per-inch (dpi) of ink jet systems. These systems are also limited in terms of the surface area of a substrate that can be printed, and in many cases these systems require a planar substrate. As with rotogravure printing, full color printing is not possible with silk screen or pad printing methods, and changeover of graphic designs is both time consuming and expensive.
Another popular method for forming images on edibles, particularly on cakes and other large substrates, involves using an edible transfer sheet which can be handled in a printer and which are relatively porous and hydrophilic and therefore can readily accept an image from a water-based ink, including an ink-jetted image. Various methods are then used to transfer the image from the sheet to an edible substrate. An exemplary ink for use with this type of system is disclosed in U.S. Patent Application Publication US 2002/0008751 and comprises water, isopropyl alcohol, sodium lauryl sulphate and FD&C colorant. In the transfer sheet technology, printing is not performed directly on the surface of an edible conveyed past a printhead. Using a transfer sheet requires a component of the substrate to partly dissolve the sheet or to allow the sheet to adhere to the substrate. Thus, the transfer sheet technology is not readily adapted to the high speed production of images on non-planar surfaces of confectionery pieces. The water-based inks adapted for use with transfer sheets do not perform properly for ink-jet printing directly on non-planar, non-porous and hydrophobic surfaces, as they adhere poorly, dry too slowly and lack opacity.
Ink-jet printing on edibles, if the technology could be perfected, would be attractive from many vantage points. It would eliminate the need to contact the edible substrate with a contact member such as a pad or roller. Further, since ink-jet printing is a non-contact printing system, slight variations in the size of edibles would not negatively impact upon printing quality, as typically occurs with pad or roller based systems. Also, an ink-jet printer image is stored as data, and not fixed on a contact member. Consequently, images could be selected, altered, transmitted, and the like, more easily than in contact printing, opening the possibility for rapid changeover of printed designs and personalized graphics.
Ink jet printing systems are broadly divided into continuous jet, and drop-on-demand (also called “impulse”) systems in which droplets are generated as needed for ejection to the substrate surface for image formation. Methods of ink-jet printing on edible substrates using continuous jet technology have been disclosed. Most of these are directed to labelling and the like applications which do not require high resolution. Examples of such prior art disclosures include: U.S. Pat. Nos. 4,168,662, 5,453,122, 5,006,362, 5,397,387 and 5,800,601.
In continuous jet systems, ink is emitted in a continuous stream under pressure through at least one nozzle. The stream is broken up into droplets at a fixed distance from the orifice, typically, by a piezoelectric crystal, which is vibrated at controlled frequency adjacent to the ink stream. This function of the piezoelectric element is different from the function of the piezoelectric element in a piezojet system, where the piezoelectric element controls the generation of droplets from a reservoir. To control the flow of ink droplets in a continuous jet system, the inks are charged (by addition of salts and other conductive agents) and the droplets are passed through an electrostatic field, which adjusts the trajectory of the droplets in accordance with data signals. The droplets are either directed back to a gutter for recirculation or to a specific location on the substrate to create the desired character matrix. A typical resolution for a continuous jet printer image in an industrial setting, using a single printhead and a single pass printing is about 75-100 dpi.
Most continuous jet inks are solvent-based, containing substantial amounts of methyl ethyl ketone (MEK) or an equivalent volatile organic compound (VOC) as the carrier, with lesser amounts of water and a lower alcohol. Inks for continuous ink jet systems are also characterized by the presence of salts, such as potassium thiocyanate, or other conductivity-enhancing agent, which enables the droplets to be deflected in an electrostatic field. Typically, the conductivity of such inks is greater than about 2000 micro Siemens. Thus, the inks that have been developed for industrial continuous jet printing are not readily adapted for printing on edibles, as they typically contain non-edible, and sometimes toxic, ingredients.
Continuous jet inks also have a very narrow range of acceptable viscosity. Inks having a viscosity above about 10 centipoise (cp) at low shear rates cause the pumps in the printhead to cavitate during use. Below a viscosity of about 2 to about 3 cp, the jets are not stable. Thus most, if not all, continuous jet ink jet inks have a viscosity of about 2.8 to about 6 cp.
Of the drop-on-demand systems, the most economically important today are piezojet and bubblejet (sometimes referred to as thermal ink jet) systems. In bubblejet systems, a bubble is formed by a resistance heater in an ink reservoir. The resulting pressure wave from the bubble forces ink through an orifice plate. Once the heat is removed, the bubble collapses and a droplet is ejected. Bubblejet printheads dominate the home and office ink-jet printer markets and they are capable of very high resolution. However, several considerations limit their use in an industrial setting, particularly their use with edible substrates.
The viscosity of bubblejet inks is very low, on the order of 1.5 cp, necessary so that a bubble can form quickly upon application of minimal voltage to the resistance heater. Moreover, the ink must be capable of withstanding the temperature cycling encountered inside the printhead. Consequently, the number of inks which could be developed for printing on edibles with a bubblejet printer is extremely limited. Bubblejet printers are also much too slow to permit high speed printing directly on edible substrates.
A method of printing on edibles is described in co-pending U.S. patent application Ser. No. 09/587,108, incorporated herein by reference. The ink described therein is a pigmented white ink which has found utility for printing on chocolate.
Piezojet ink-jet systems are commercially available in which the ink undergoes a phase change from a solid state to a liquid state in the printhead. However, edible wax-based inks for such printheads are not commercially available.
Thus, there continues to be a need in the art for inks which adhere better to the known edible substrates and which at the same time have properties which render them suitable for use with existing piezojet ink-jet printheads. In particular it would be desirable to develop an ink for effective printing on hydrophobic surfaces, which cause water based inks to bead up or smear. Such inks have been developed after careful consideration of the properties desired in such inks and demanded by the physical constraints of the known printheads.